High speed impact printers, as well as other devices, utilize electromagnetic actuators which impel an impact element, commonly called a hammer, against moving type to produce printing. The impact element can be either a ballistic or driven member. The actuators are electromagnets comprising a coil energized by an electric driver circuit which controls the application of current from a DC power source to the coil. High performance impact printers require tight control of the energy imparted to the hammers to obtain good print quality. Variations in energy, resulting from variations in the voltage of the power source or by temperature induced changes in the resistive properties of the coil, affect flight time and impact force level causing undesirable variations in the appearance of the resultant printing.
One approach to solving the problem is to regulate the power source to maintain a constant voltage. However, regulated power supplies may be more costly than desired. Also this would not correct for coil temperature effects. Another approach uses chopper type drivers which can provide good control of the steady state current over a large range of power supply voltages but do not precisely control the total energy for a given time period. One type of chopper drive circuit is the shown in U.S. Pat. No. 4,381,532, where the amount of energy is controlled by monitoring the power supply voltage and by chopping the steady state component of the drive current in the coil at a frequency related to the magnitude of the power supply voltage. In U.S. Pat. No. 4,408,129, the circuit chops the current during the rise time portion as well as the steady state portion operative interval. While this circuit controls the energy supplied to the coil during both the rise time and steady state portions of the operative interval, two chopping circuits are used, one of which is dependent on analog components including a tight tolerance capacitor and a precision current source for generating a reference slope.
Other examples of drive circuits with means for controlling the amount of energy supplied to the coil of an electromagnet may be seen in U.S. Pat. Nos. 4,293,888; 4,503,480; 4,667,117 and in the IBM Technical Disclosure Bulletin Vol. 22 No. 5, of October, 1979, pp. 1756-1758 and Vol. 24 No. 11B of April, 1982, pp. 6105-6106. All of these compensate for variations in voltage occurring during the turn on time of the electromagnet by adjusting the turn on period depending on when a predetermined current level is reached. This produces a hammer firing period which is variable which in some cases complicates the timing of other printing operations or is necessarily limited to avoid adverse impact on other timing operations.